The two sides knew little of each other before the war, and little after the war. Based on the limited state of knowledge that still existed in as late as 2266, the war was summarized as: "Earth believes the Romulans to be warlike, cruel, treacherous; and only the Romulans know what they think of Earth." (TOSepisode: "Balance of Terror".)

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V'Las, Administrator of the High Command of the Confederacy of Vulcan and clandestine military intelligence officer for the Romulan Star Empire.

The Romulan War's origins lay in the program of covert expansionism adopted by the Romulan Star Empire in the mid-22nd Century. Throughout that century, the Empire had adopted a program of unlimited territorial expansion and conquest so widely accepted that when then-SenatorValdore i'Kaleh tr'Ihaimehn sought to publicly question it, he was expelled from the Senate and faced execution before fellow Senator and old friend Vrax intervened with the First Consul to have the Praetor spare Valdore's life. (ENTepisode: "The Aenar"; ENTnovel: The Good That Men Do)

Undeterred, the Romulans immediately began a new program of covert destabilization under the leadership of Valdore i'Kaleh tr'Ihaimehn, now an admiral in the Romulan Imperial Fleet. Seeking to instigate a war between Andoria and Tellar, the Romulans began using remotely-controlled drone ships capable of disguising themselves as foreign vessels to attack Andorian and Tellarite ships, forcing captured Aenar to control the ships. In November of that year, however, Archer again thwarted their plans, uncovering evidence of Romulan involvement and unifying the Vulcan, Andorian, Tellarite, and Earth fleets against the drone ships. This helped lay the foundation for United Earth Prime MinisterNathan Samuels's creation of the Coalition of Planets the following year. (ENTepisodes: "Babel One", "United", "The Aenar", "Demons")

Upon the founding of the Coalition, the Romulans stepped up their efforts against the allied worlds. Launching a sneak attack against the People's Republic of Coridan in order to prevent its vast dilithium resources from falling into Coalition hands, the Star Empire sent a small spacecraft to collide with Coridan Prime at warp speeds in February 2155, creating a devastating impact that immediately killed nearly half a billion Coridanites and consumed much of the dilithium mines in a planetary fireball. While Captain Archer insisted that the Romulans were behind the attack, neither the Coridani or Coalition governments were able to ascertain this. As a result of the attacks, ChancellorKalev withdrew the People's Republic of Coridan from the signing of the Coalition Compact. (ENTnovel: The Good That Men Do)

By mid-2155, the Imperial Fleet began using a sophisticated computer virus to remotely control various foreign vessels. Using this virus, Valdore's forces used hijacked Klingon starships to launch attacks on Alpha Centauri, allied with Draylax, and upon a commercial ship carrying equipment and personnel for a joint Vulcan-Earth spy station on Sataghni II , the SS Kobayashi Maru, destroying it near the Tezel-Oroko system in spite of the attempted intervention of the United Earth starship Enterprise NX-01. The Romulans also used hijacked Vulcan ships to launch an attack on Centauran freighters. This was part of a continual program to foment astropolitical chaos as a prelude to invasion, attempting to manipulate the Coalition of Planets into going to war with the Klingon Empire while undermining the trust amongst the Coalition members. (ENTnovel: Kobayashi Maru)

On July 22, 2155, Romulan telecaptured Klingon ships destroyed and tried to remotely hijack Vulcan ships near Alpha Centauri. A similar act was taken in the Gamma Hydra sector with three telecaptured D5 class Klingon ships attempting to remotely hijack the Earth starship Enterprise NX-01 and destroy the SS Kobayashi Maru. There would also be a Romulan invasion force sent to Calder II to take the planet and establish a beachhead. (ENTnovels: Kobayashi Maru, Beneath the Raptor's Wing)

On Stardate 1/0101.01 during the Century Day celebrations on Luna and other colonies, news was received that ended the event which stated that 3 heavy cruisers were attacked and presumed lost to an unknown enemy. A single destroyer attached to the fleet had, however, survived and reported of a sneak attack by vessels painted with a giant winged creature. The unknown enemy were responsible for performing suicide runs with fusion explosive missiles. Intercepted subspace radio transmissions from the battle were later translated and revealed that their enemy had a name; Romulan and also gave an indicated on the first bearings of Romulan held space.

Between Stardate 1/0101 to 1/0601, there were several more skirmishes between the Romulans and their opponents which led to Starfleet beginning to gain some idea on how to deal with Romulan hit-and-run tactics. On Stardate 1/0522.07, two Romulan vessels were crippled by a Starfleet strike force who began boarding actions. The Romulan commanders waited until the ships maneuvered close enough for grappling and boarding action to commence after which the Romulans initiated their self destruct taking out the Starfleet ships with them as they exploded. After this chain of events, Starfleet issued orders that prohibited any vessel from approaching even a badly damaged Romulan vessel. (FASA RPGmodule: The Romulans (FASA))

In 2156, a Romulan ship attacked the United Earth starship Columbia NX-02 in deep space. In the course of the attack, the Romulan ship was destroyed, and Columbia severely damaged and was not able to contact Earth to warn of the Romulan attack. The Columbia's commander to set course for the planet Erigol, using the time dilation effect of very high impulse speeds to get there in a few years, without the ship's warp engines functioning. As a result the Columbia missed the Romulan War, which began shortly after this attack on ship. The Columbia's ultimate fate would not be discovered for more than two centuries. (ST - Destinynovel: Gods of Night)

On May 18, 2156, a Romulan fleet consisting of five ships attacked Starbase 1 in the Algeron system, destroying the base and two NX class starships assigned there, Columbia and Excalibur. While Admiral Gardner immediately wanted to declare war, Starfleet Command felt differently and asked for his resignation. They replaced him with Admiral Rafael Douglas and sent a message to the Romulans, demanding an explanation for the attack. This message further convinced the Romulans of Earth's weakness. (STreference: Federation: The First 150 Years)

The relationship between the Earth Alliance and the Coalition of Planets – including the possibility of the Earth Alliance being an informal name for the Coalition of Planets – remains not established.

On Stardate 1/0811, a Starfleet group stumbled upon a major Romulan base which dealt the enemy its first true defeat when 30 Romulan war vessels that were manned only by skeleton crews during a resupply operation were destroyed in orbit before they could fire a single shot. Six Starfleet vessels were lost in the engagement when the planetary base destroyed itself after its shields had been damaged seriously. The loss of the fleet was a crippling blow to the Romulans who never recovered fully from its effects. (FASA RPGmodule: The Romulans)

The Romulans intended to move on the Sol system and win in a sweeping victory. As a staging ground, they chose Denobula, which was officially neutral in the war and, the Romulans estimated, would not be expecting an attack. On October 1, 2158, the Romulans arrived and killed the crew of a Denobulan orbital station before firing on the planet. As they expected, the Denobulans offered no resistance. The Romulans used the Denobulan facilities to prepare their vessels for battle.

From there, they launched their attack on the Sol system with three targets: First the Verteron Array on Mars, then the Utopia Planitia Fleet Yards also on Mars, then Earth itself. They came out of warp very close to the Verteron Array, quickly destroying it. They then fired on the shipyards, unaware that all six warp-5 ships being constructed there had been evacuated during the occupation of Denobula. A second Verteron Array had been hidden within the atmosphere of Venus, which now opened fire on the Romulans, destroying two of their ships. The fleet hid behind Mars, where a Starfleet battalion lead by Bryce Shumar aboard the Intrepid came out of warp and engaged them. Six of the ten Romulan ships were destroyed before Mindar ordered a retreat. (STreference: Federation: The First 150 Years)

The Romulans came very close to taking Earth. However, the threat of invasion made Earth and its allies even more determined to defeat the Romulans and they began to slowly push them back into Romulan territory. (TOS - My Brother's Keepernovel: Republic)

The United Earth ship Enterprise NX-01, one of the principal vessels of the Earth Alliance.

During 2159 the Romulans allied themselves with the VulcanT'Uerell who "went missing", prompting the United Earth Starfleet to send Enterprise to investigate and Coto to respond to her general distress call. These ships destroyed their Romulan counterparts and escorted T'Urell to her outpost. (STvideo game: Legacy)

The Battle of Cheron is generally considered the decisive battle in the Earth-Romulan War. Both sides recognized the major victory achieved here by the Earth Alliance as a humiliating defeat of the Romulan military. This event would establish long-term political effects within the Romulan Empire lasting well into the 24th century. (TNGepisode: "The Defector"; ENTnovel: To Brave the Storm)

While Earth and its allies claimed victory at Cheron, the Earth-Romulus War ultimately led to a bloody and bitter stalemate after almost five years, with which neither side able claim victory or willing to concede defeat.

In 2266, Spock categorized the weaponry generally used in the Earth-Romulan War as atomic in nature, and fairly primitive at that. He also characterized the ships in use as primitive, and designed in ways that did not permit quarter or captives. Romulan starships were painted to resemble a "bird of prey", a tradition kept through the mid-23rd century. Ship-to-ship visual communication between Earth-allied ships and Romulan-allied ships did not take place during the war or during the peace negotiations. As a result, until the 2266 altercation between USS Enterprise and ChR Gal Gath'thong, the Federation had no idea what the Romulans looked like. (TOSepisode: "Balance of Terror")

According to some reports, the war ended inconclusively due to staggering losses sustained by both sides. By Stardate 1/0909, neither side was capable of supplying adequately or even maintain their fleets in the war zone. Starfleet was also not capable of mounting a strong enough fleet to penetrate Romulan space an reach the supposed location of the Romulan home world. This led to Senior Councilman Abraham Dannon attempting to convince his colleagues of offering a peace to the Romulans; a solution that passed by a 1-vote margin. (FASA RPGmodule: The Romulans)

From a 23rd century Human perspective, the Neutral Zone represented an area "between planets Romulus and Remus and the rest of the galaxy..." Some Humans still harbored resentment over the war after over a century of peace, including Lieutenant Stiles of the USS Enterprise.

Despite these human attitudes, until 2266 the Neutral Zone served its purpose and the treaty had remained unbroken. In fact, one might say it served its purpose too well, as there were limited or no contacts between Earth and the Romulan Star Empire from the conclusion of the war until the mid-23rd century. (TOSepisode: "Balance of Terror")

Orion forces remained out of the conflict, but a large quantity of war materiel was freighted through Orion worlds. In particular, the Orion Colony of Votannis served as a valuable rear staging area. Starfleet built high quality star port facilities here for this reason. Many Starfleet crews took leave on Orion worlds and first discovered Orion pleasures and leisure activities. (FASA RPGmodule: The Orions: Book of Common Knowledge)

The Earth-Romulan War has been portrayed several different ways by different sources.

The Romulan Way states that the Earth-Romulan War was initially fought between Starfleet exploration ships and Romulan sublight vessels, after the Romulans, then a single-system polity without warp drive, mistook the explorers as scouts for an invasion (based on their historical bad experience with the Duthulhivpirates prior to the Sundering). They captured the next Federation ship to approach Romulus and rapidly reverse-engineered it, thereby acquiring warp drive among other technologies. The war then escalated until the Romulans were destroying entire fleets, at which point Starfleet Command decided to cut its losses and negotiated a peace treaty via subspace radio, designating a swathe of space surrounding the Romulan system as the Romulan Star Empire. This section of the novel was almost entirely contradicted by Star Trek: Enterprise, which aired fourteen years after the book was published and established the Romulans as already having warp drive and an interstellar state well before the Federation's founding.

The FASA RPG line states the war began between the UFP and the Romulans 19 years after its founding, lasting from reference stardate 1/0610 to 1/0909. This contradicts canonical elements from the show, as an illegible viewscreen graphic seen aboard the USS Defiant in the ENT episode "In a Mirror, Darkly, Part II" lists the conflict as beginning in 2156 and ending in 2160. The novelization of Balance of Terror places it later- starting in 2191 and lasting 25 years. It also establishes the neutral zone as a sphere around the Romulus/Remus system unlike how it is portrayed in other sources.

The ENT relaunch novels were partially based on notes for the fifth through seventh seasons of the show written prior to its cancellation, making them the intended canonical version. The Good That Men Do states that the Earth-Romulan war began in 2156, some time after the incident with Terra Prime, and ended in 2160. The novel also suggests that the Federation was born from the conflict. The subsequent miniseries The Romulan War confirms this and portrays the actual conflict.